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Mineralogical, gemmological and petrological study of the Mogok Stone Tract in Myanmar with a special focus on gem-quality ruby and spinel

Phyo, Myint Myat. Mineralogical, gemmological and petrological study of the Mogok Stone Tract in Myanmar with a special focus on gem-quality ruby and spinel. 2019, Doctoral Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_13306

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Abstract

This thesis mainly focuses on the gemstones (marble-hosted ruby and spinel) from world famous ruby deposits in the Mogok area, Myanmar. This dissertation consists of three main sections (one published paper and two drafts for publications to be submitted): (1) Spinel from Mogok, Myanmar - a detalied inclusion study by Raman microspectroscopy and scanning electron microscopy, (2) Petrology, geothermobarometry and geochemistry of granulite facies gneisses in the vicinity of gemstone deposits from Mogok area (Myanmar) and (3) U-Pb age dating of zircon and zirconolite inclusions in marble hosted gem-quality ruby and spinel from Mogok, Myanmar.
During the last few years, spinel became one of the most popular gemstones and currently is highly demanded in the international gem market. Therefore, it was my intention to carry out a detailed inclusion study on Mogok spinel which might become in the future an essential tool for gem laboratories to enable conclusive origin determination analyses of spinel. For this research, Raman microspectroscopy and scanning electron microscopy (SEM) analyses were carried out to identify the inclusions in Mogok spinel. Based on my research, I was able to document 16 mineral inclusions in Mogok spinel so far not known in literature. Furthermore, these studies confirmed that most solid inclusions in Mogok spinel are in fact multi-phase inclusions which may contain both, primary and secondary mineral phases (such as dolomite, apatite, anhydrite, phlogopite, and halite in calcite).
Three garnet-quartz gneisses, three peculiar garnet-clinopyroxene-nepheline gneisses and one clinopyroxene-clinoamphibole gneiss were selected for geothermobarometric studies. Conventional geothermobarometry of the garnet-quartz gneisses revealed granulite facies PT-conditions of 758-798°C at 7.2-7.6 kbar. The nepheline-gneisses and the clinopyroxene-clinoamphibole gneiss were also metamorphosed at such high-grade metamorphic conditions. Equilibrium phase diagram calculations with the Theriak-Domino program reproduced these PT-estimates and pointed at water activities of 0.34-0.4. Whole rocks geochemistry, trace elements analyses and isotope studies were carried out on the garnet-clinopyroxene-nepheline gneisses and the clinopyroxene-clinoamphibole gneiss. The geochemical investigations highlighted the high-K calcalkaline and shoshonitic character of these foid-bearing rocks pointing to a strong fractionation and to a subduction-related generation of their magmatic protoliths.
For the geochronology (U-Pb age dating) study, I analysed a selected number of zircon and zirconolite inclusions from ruby and spinel samples, and from ruby-and spinel-bearing marble as well as from gneisses (two garnet-quartz gneisses), all from the Mogok area. Two different setups of Laser Ablation Inductively Coupled Plasma Mass Spectrometers were used, (a) Time-Of-Flight and (b) Sector-Field mass spectrometer to detect the U-Pb isotopes in these inclusions. The calculated U-Pb ages in zircon inclusions from gemstones as well as from gneisses indicate that the Mogok area experienced several magmatic and metamorphic events. Specifically, the U-Pb ages of zircon from gem-quality corundum, spinel and marble revealed maximum ages of Late Cretaceous in their cores and minimum ages of Late Oligocene to Early Miocene in the rim of zircon grains.
Garnet-quartz gneiss from the western part of the area revealed significant lead loss conditions and U-Pb ages ranging from Precambrian to Jurassic age in the core, whereas the rim again revealed Oligocene ages. The garnet-quartz gneiss from central Mogok showed Middle Jurassic to Early Cretaceous ages in the zircon core and Oligocene ages in the rim of zircon grains. I assume that the Precambrian age of zircon in gneiss is probably representing the so-called Mogok gneiss of Iyer (1953).
Advisors:De Capitani, Christian E. and Krzemnicki, Michael Stanislaus and Franz, Leander
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Mineralogie-Petrographie > Theoretische Petrologie (De Capitani)
UniBasel Contributors:De Capitani, Christian E.
Item Type:Thesis
Thesis Subtype:Doctoral Thesis
Thesis no:13306
Thesis status:Complete
Bibsysno:Link to catalogue
Number of Pages:1 Online-Ressource (xv, 178 Seiten)
Language:English
Identification Number:
Last Modified:14 Nov 2019 05:30
Deposited On:13 Nov 2019 13:44

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